The present invention relates to a resin composition for forming a durable protection coating, which may be used as a solder resist, a marking or a solder leveller, on a substrate such as a printed circuit board, and to a process for forming such a durable protection coating or layer by applying the aforementioned resin composition on the substrate followed by curing.
As the integration degree of printed circuit boards progressively advances higher and higher from year to year, it is required in recent years to draw three or even more lines through respective zones between adjacent IC lands. Accordingly, it becomes necessary to provide a durable protection coating which serves, for example, as a solder resist and is capable of accommodating such a high integration degree to ensure that the circuit board has satisfactory reliability. However, when preparing such a durable protection coating by the use of a conventional thermosetting resin compositions, since a pattern must be formed through silk screen printing, there often arises a problem that a fine pattern cannot be formed due to dislocation caused by slacking of the silk screen. Ultraviolet ray curing type compositions capable of forming fine patterns have been frequently used up to date. On the other hand, although compositions which can be developed by various organic solvents have heretofore been used, in order to improve the working environment or in order to simplify and lessen the size of entire installation, organic solvent developed type compositions have been gradually replaced by resin compositions developable with alkalis. Such replacement is also desirable for saving resources and energies with attendant improvement in operation efficiency and producibility.
As should be understood from the foregoing, resin compositions which are capable of forming superfine or micro patterns upon curing by exposure to ultraviolet ray, and then developed with an alkali, are the most preferable materials for the formation of durable protection layers or coatings. Development of such resin compositions is thus earnestly desired. However, such a resin composition, which can be cured upon exposure to ultraviolet ray and developed with an alkali to form a durable or lasting protection coating while satisfying the required adaptability to higher degree integration of the circuit pattern and having satisfactory reliability, has not yet been found. Some reasons therefor would be noticed, the first reason being that the resin compositions developed with an alkali has a disadvantage that the carboxylic acid groups which are indispensable for the development with an alkali remain in the resultant coating after it is formed, leading to the result that the formed coating has poor moistureproof properties, insufficient resistance to chemicals and hydrolysis and unsatisfactory adhesiveness. The more serious problem induced by the known resin compositions, when they are applied on a printed circuit board, is that carboxylic acid groups in the resin corrode a copper foil forming the printed circuit to deteriorate the properties of the substrate. In order to obviate such problems, it is reluctantly compelled to use resins each containing an acid value of not more than about 50 mg KOH/g, resulting in unsatisfactory resolving power of the resin after exposure and subsequent development thereof. On the other hand, resin compositions commercially available today have disadvantages that they are unsatisfactory in resistance to chemicals and solvents and have low reliability when they are cured in the form of thick coatings. When a durable or lasting protection coating is prepared by using a resin composition which is cured by exposure to ultraviolet ray, since a circuit pattern forming mask must be closely fitted over the resin coating at the step of exposing the coating to ultraviolet ray, it is requisite that the coating should not have tack or stickiness. More particularly, it is necessary that the coating does not exhibit tack by the finger touch test at ambient temperature before it is exposed to light.